A surface acoustic wave device, which is an electromechanical transducer device utilizing surface waves propagated on the surface of an elastic body, has a general structure shown in FIG. 1, for example. In such a surface acoustic wave device 30, a piezoelectric phenomenon which is caused by a piezoelectric member 31 is utilized for exciting surface waves. When an electric signal is applied to interdigital electrodes 32 which are provided on the piezoelectric member 31, the piezoelectric member 31 is distorted to generate surface acoustic waves, which in turn are propagated along the piezoelectric member 31 and received in other interdigital electrodes 33 as an electric signal. The term "interdigital" as used herein means that the electrodes have finger-like electrode elements which are positioned to intermesh as shown in FIG. 1. Assuming that .lambda..sub.0 represents the electrode period in the interdigital electrodes 32 and 33 and v represents the velocity of the surface acoustic waves in relation to frequency characteristics, this device has a bandpass characteristic with a center frequency f.sub.0, which is expressed as f.sub.0 =v/.lambda..sub.0.
The surface acoustic wave device can be miniaturized with a small number of components, while it is possible to input/output signals on a surface wave propagation path. This device can be used as a filter, a delay line, an oscillator, a resonator, a convolver, a correlator etc. In particular, a surface acoustic wave filter has been practically used as an intermediate frequency filter for television over a long time, while the same is now being used as filters for VTR and various communication devices such as a portable telephone and a car telephone.
In relation to the aforementioned surface acoustic wave device, a device useful in a higher frequency range of 1.5 to 3 GHz with regard to official frequency allocations is desirable as a surface acoustic wave filter for use in the field of mobile communication, for example. As understood from the above expression, such a device has a frequency characteristics with higher center frequency f.sub.0 if the electrode period .lambda..sub.0 is reduced or the velocity v of the surface waves is increased.
Japanese Patent Publication No. 54-38874 (1979) discloses a surface acoustic wave device which comprises a substrate, a piezoelectric thin film and an insulating layer, made of a material (e.g., sapphire) having a larger surface acoustic wave propagation velocity than that in the piezoelectric thin film, provided between the substrate and the piezoelectric thin film. On the other hand, Japanese Patent Laying-Open No. 64-62911 (1989) in the name of the assignee of the present application discloses a surface acoustic wave device comprising a piezoelectric member which is stacked on a diamond layer for increasing the velocity v of surface acoustic waves. Diamond has the largest sound velocity among materials, and the same is physically and chemically stable.
Each of the aforementioned surface acoustic wave devices has been generally stored in a package, with no passivation of its center portion which is defined by a pair of interdigital electrodes and a piezoelectric member. This is because the characteristics of the device, particularly the frequency characteristics, are inevitably changed if an insulating film for passivation is provided on the piezoelectric member for propagating surface acoustic waves. However, a device packaged without any passivation cannot be sufficiently guaranteed in reliability. When such a non-protected surface acoustic wave device is surface-mounted on an electronic device to be connected to a peripheral circuit such as an amplifying circuit, it is necessary to package the device to avoid damage to the piezoelectric member and to the interdigital electrodes. In practice, however, it has been impossible to carry out such packaging, due to the uncertain reliability of the surface acoustic wave device whose surface is not passivated.